Hello and welcome!

Check out my very first Post Top fixture. I'm not sure of the brand but it was made in 1980 and it has a H43 75 watt ballast. Unfortunately the bulb is missing and I think the ballast is done. Only measuring 124v at the socket when the power is applied. OCV isn't high enough? or??
 

I stand corrected! Another member suggested that the ballast might be disconnected and sure enough it was.

Someone had bundled all three of the ballast wires together and I didn't even clue in!

Here's a pic of the 75w Mercury Vapor ballast. No markings that I can see anywhere. I really have no idea what wires are what.
I'm assuming a 120V Hot in and out and a Comm?
 

I stand corrected! Another member suggested that the ballast might be disconnected and sure enough it was.

Someone had bundled all three of the ballast wires together and I didn't even clue in!

Here's a pic of the 75w Mercury Vapor ballast. No markings that I can see anywhere. I really have no idea what wires are what.
I'm assuming a 120V Hot in and out and a Comm?
 

That is a choke type ballast. Never seen that for MV.

There are 3 wires and what looks like Magnetic shunt, so it is a HX



Trying to guess which wire is which (im not really sure, somebody confirm ?) :

 - The 2 wires coming from the same coil are probably the line supply - call them X Y, the one coming from other coil probably to the lamp - call it L

 - If this guess is correct, then you can plug X Y into 120V (does not matter yet who is Phase or Neutral)

 - Now measure voltages X-L and Y-L. The one that got higher voltage, is the one from Neutral : If X-L voltage is higher, then X is Neutral. If Y-L higher, then Y is Neutral

 - And now you can connect X Y to Phase and Neutral of the 120V in the correct polarity, and the lamp between L and Neutral

The top right wire has a black fabric cover if you peel back the outer fabric insulation. I made all the connections and the ballast fired up the 100w A23 MV bulb just fine. Is .77 amps to much draw for this small of ballast?

I dunno about MV, but a 100w HPS NPF Reactor ballast draws about 3.2 amps.  My 250w Mercury HX-HPF NEMA head draws 8.2amps

Sweeeet! Is there a capacitor with the ballast? If so, what's the value?

I dunno about MV, but a 100w HPS NPF Reactor ballast draws about 3.2 amps.  My 250w Mercury HX-HPF NEMA head draws 8.2amps

8.2 Amps, that seems abit high for a 250w MV.

Sweeeet! Is there a capacitor with the ballast? If so, what's the value?

No capacitor installed in this fixture or ballast. Not sure why.

Because it is low power factor. Such lanterns are usually intended for installation in low quantities (private property with handfull of lanterns), so that the higher current from each lantern dont sum up to huge currents for a big setup

So if I'm understanding the theory behind capacitor use in A/C powered circuits, these are used to help with voltage drops across a load when many loads (light fixtures, electric motors etc) are connected in parallel on the same circuit?

They are used for power factor correction. The closer the power factor of the device to unity, the less current the device draws for performing the same work. Current is a flow of charges (Electrons), each of which is carrying Energy. So, electrical current is responsible to moving power (from the power source to the device using it for example)

At power factor of unity, the device draws the minimum needed current. The current going is exactly the minimum it takes to get there the power required for the device :

I(current) = P(power) / V(voltage)

At lower power factor, the deice draws more current, so more power comes in. Some of it (how much ? - That is the power factor) is what is actually needed by the device (active power). The rest is coming in, stored inside the device for a while (quarter of the AC cycle), and sent back out into the supply (reactive power). So this power is not used by the device, is not wasted either, but is carried back and forth through the wiring for nothing

This happens with devices that can stores some Energy and discharge it later :



Coils :



In coil the current lags after the voltage in time. That is, when the voltage (Blue) is going up, the current (Red) is still in the down peak. When the voltage allready reached the up peak, the current is still going up. And so on

The Energy is stored in the coil in the form of Magnetic flux, and that is related to the current : The Energy is getting in and stored as the current goes up (towards the positive or negative peak), and discharged out as the current goes down (from the peak to 0)

The power of the coil is :



Power is positive when Energy is getting in, and negative when it is getting out



Capacitors :



In coil the current leads before the voltage in time. That is, when the voltage (Blue) is going up, the current (Red) is allready at the up peak. When the voltage reached the up peak, the current is allredy going down. And so on

The Energy is stored in the capacitor in the form of Electric charges, and that is related to the voltage : The Energy is getting in and stored as the voltage goes up (towards the positive or negative peak), and discharged out as the current goes down (from the peak to 0)

The power of the capacitor is :



Power is positive when Energy is getting in, and negative when it is getting out



Their behavior is opposite, that is when one is discharging the other is charging. If we connect them together, then some of the reactive power (as much as fits in) will go back and forth between them, and only the remaining part go out to the supply line

If we choose them so, that they store the same Energy, then the reactive power will entirely go between them, and not be present on the supplying line. Connecting a capacitor of the matching value to a coil circuit (ballast, motor, ..) is how power factor correction is done : We know what is the reactive power of the ballast/motor/.. that we want to correct, and choose capacitor that stores the same power



As the current through the supply line is lower with power factor correction (without the reactive current going throught there) :

 - Lower losses on the line, so some energy savings (allthough the device in the end still uses the same power as before, we save what was otherwise wasted as heating of the wiring)

 - Lower voltage drop on the line

 - Can connect more of the same devices to one circuit (example 10 Mercury lanterns with PFC instead of 5 without PFC, on one 15Amp circuit)

That is why for the home user (who have at most a handfull of the lanterns on his property, or handfull of Fluorescents inside the house) it does not matter that much, but for those who have 10's or 100's of the lanterns on the same circuit, it does

In addition, the power companies impose requirements on commercial/industrial users to keep the power factor up (corrected), to prevent losses/overloading on their powerlines caused by user's equipment. So along with power factor correction systems for big motors and such, those users choose HPF lighting to comply with this requirement too

Home users dont have lots of the same luminaires or big motors like commercial/industrial users, they have all many different small/relatively small appliances, so the power companies dont bother with them

They are used for power factor correction. The closer the power factor of the device to unity, the less current the device draws for performing the same work. Current is a flow of charges (Electrons), each of which is carrying Energy. So, electrical current is responsible to moving power (from the power source to the device using it for example)

At power factor of unity, the device draws the minimum needed current. The current going is exactly the minimum it takes to get there the power required for the device :

I(current) = P(power) / V(voltage)

At lower power factor, the deice draws more current, so more power comes in. Some of it (how much ? - That is the power factor) is what is actually needed by the device (active power). The rest is coming in, stored inside the device for a while (quarter of the AC cycle), and sent back out into the supply (reactive power). So this power is not used by the device, is not wasted either, but is carried back and forth through the wiring for nothing

This happens with devices that can stores some Energy and discharge it later :



Coils :



In coil the current lags after the voltage in time. That is, when the voltage (Blue) is going up, the current (Red) is still in the down peak. When the voltage allready reached the up peak, the current is still going up. And so on

The Energy is stored in the coil in the form of Magnetic flux, and that is related to the current : The Energy is getting in and stored as the current goes up (towards the positive or negative peak), and discharged out as the current goes down (from the peak to 0)

The power of the coil is :



Power is positive when Energy is getting in, and negative when it is getting out



Capacitors :



In coil the current leads before the voltage in time. That is, when the voltage (Blue) is going up, the current (Red) is allready at the up peak. When the voltage reached the up peak, the current is allredy going down. And so on

The Energy is stored in the capacitor in the form of Electric charges, and that is related to the voltage : The Energy is getting in and stored as the voltage goes up (towards the positive or negative peak), and discharged out as the current goes down (from the peak to 0)

The power of the capacitor is :



Power is positive when Energy is getting in, and negative when it is getting out



Their behavior is opposite, that is when one is discharging the other is charging. If we connect them together, then some of the reactive power (as much as fits in) will go back and forth between them, and only the remaining part go out to the supply line

If we choose them so, that they store the same Energy, then the reactive power will entirely go between them, and not be present on the supplying line. Connecting a capacitor of the matching value to a coil circuit (ballast, motor, ..) is how power factor correction is done : We know what is the reactive power of the ballast/motor/.. that we want to correct, and choose capacitor that stores the same power



As the current through the supply line is lower with power factor correction (without the reactive current going throught there) :

 - Lower losses on the line, so some energy savings (allthough the device in the end still uses the same power as before, we save what was otherwise wasted as heating of the wiring)

 - Lower voltage drop on the line

 - Can connect more of the same devices to one circuit (example 10 Mercury lanterns with PFC instead of 5 without PFC, on one 15Amp circuit)

That is why for the home user (who have at most a handfull of the lanterns on his property, or handfull of Fluorescents inside the house) it does not matter that much, but for those who have 10's or 100's of the lanterns on the same circuit, it does

In addition, the power companies impose requirements on commercial/industrial users to keep the power factor up (corrected), to prevent losses/overloading on their powerlines caused by user's equipment. So along with power factor correction systems for big motors and such, those users choose HPF lighting to comply with this requirement too

Home users dont have lots of the same luminaires or big motors like commercial/industrial users, they have all many different small/relatively small appliances, so the power companies dont bother with them

Thanks for the very informative and technical write up. It's pretty interesting how the correctly selected capacitor can alter  the behavior of A/C current and voltage. Here is a few pics of my Regent mini bucket light that I'm going to convert to 50w MV
As you can see the space is not very large.